1. Field of the Invention
The present invention relates to a T-shaped film extrusion die for use in forming a film of a synthetic resin and, more particularly, to a T-shaped film extrusion die which can automatically adjust the thickness of a resin film with high precision within a short period of time.
2. Description of the Prior Art
There are two methods for forming resin films such as polyethylene or polypropylene films, one being a T-shaped die method and the other an inflation method. Of these two methods, the T-shaped die method has been widely used for manufacturing a material for resin bags for fertilizers, bags for packaging, moisture-proof bags or the like by forming a polyethylene, polypropylene, or ethylene-vinyl acetate film by extrusion and laminating the hot resin film formed by extrusion with a metal foil, Cellophane paper, kraft paper or the like and then pressing the whole together by a roller.
Therefore, the resin film obtained by the T-shaped film extrusion die (T-shaped die) is required to have a uniform thickness.
In a conventional T-shaped die, the thickness of a resin film is adjusted as follows. The width between lips of the T-shaped die (distance between the lips: lip-width) as the width of a resin discharge orifice thereof is manually adjusted by a plurality of bolts (lip-width adjusting bolts) arranged along the widthwise direction of the T-shaped die to obtain a resin film having a desired thickness. Thereafter, a resin film is then formed by extrusion at a predetermined temperature, and is wound into a roll shape. The thicknesses of respective portions of the roll are measured to calculate a three-dimensional pattern (unevenness in thickness) of the resin film, i.e., a deviation in thickness. According to thus obtained data, resin temperatures at respective portions in the T-shaped die are adjusted by a plurality of heaters arranged along the widthwise direction of the T-shaped die, thereby correcting the deviation in thickness. However, adjustment of the deviation in thickness based on the resin temperature cannot be quickly performed with high precision since adjustment of resin temperature takes much time.
Since the take-up speed of a resin film is as high as 100 m/min, a resin film having a nonuniform thickness wound during this interval (the time required for said adjustment of the deviation in thickness) is wasted, resulting in a disadvantage in terms of economy.
In addition to above thickness adjustment at the beginning of resin film formation using a T-shaped die, the thickness of the resin film must be corrected during its operation. More specifically, since four to five lip-width adjusting bolts fall within a temperature control range of one heater, if a deviation in thickness of the film falls within the range of one heater, the lip-width must be finely adjusted (corrected) by the lip-width adjusting bolts. Fine adjustment of the lip-width, heretofore, must be performed after the formation of a resin film by extrusion is interrupted. For this reason, the formation of a resin film must be interrupted for 30 minutes or more, resulting in a considerable time loss.
Further, since there are approximately 30 to 60 lip-width adjusting bolts in a T-shaped die, it is inefficient and cumbersome to manually adjust the bolts In addition, since the T-shaped die is heated to a high temperature during its operation, it is dangerous to adjust the bolts, resulting in very poor operativeness (operation efficiency) of the T-shaped die used.
As described above, it takes much time to adjust the thickness of a resin film in a conventional T-shaped die, and it is difficult to desirably control the thickness of the resin film as a product In addition, when a conventional T-shaped die is used to form a resin film, a large amount of a resin film having a nonuniform thickness, which cannot be used as a product, is formed, resulting in a disadvantage in terms of economy.